Concrete has the largest production of all man made materials. Compared with other construction materials, it possesses many advantages including low cost, general availability of raw materials, adaptability and utilization under different environmental conditions. Therefore, concrete will most probably continue to be the dominant construction material in the foreseeable future. Unfortunately, plain concrete is also a brittle material with very low tensile strength and strain capacity, generally requiring reinforcement before it can be used extensively as a construction material.
The idea of using another material to reinforce a low tensile strength material is an age-old concept. For example, straw and horsehair have been used for thousands of years to improve the properties of clay bricks. Bentur, A, Mindess, S., "Fiber Reinforced Cementitious Composites," (Elsevier Applied Science, 1990). In more recent years, fibers have been incorporated into a wide range of engineering materials (including ceramics, plastics, cement, and gypsum products) to enhance the performance properties of the resulting composite. American Concrete Institute ACI 544.1R-96, "Fiber Reinforced Concrete," 1996. Introduction of fibers into concrete results in post-elastic property changes that range from subtle to substantial, depending upon a number of factors, including matrix strength, fiber type, fiber modulus, fiber aspect ratio, fiber strength, fiber surface bonding characteristics, fiber content, fiber orientation, aggregate size effects, and the like. The enhanced properties include tensile strength, compressive strength, elastic modulus, crack resistance, crack control, durability, fatigue life, resistance to impact and abrasion, shrinkage, expansion, thermal characteristics, and fire resistance. Id.
Although fiber reinforcement is known to be more effective than conventional reinforcement in the control of local cracking, unlike reinforcing bars, it does not, in most cases, provide any increase in the load bearing capacity of concrete. Conventional reinforcing bars are strategically located in the structure to carry tensile stresses while fibers are distributed randomly in the concrete mire. Fibers, therefore, are not used in design as a substitute for conventional reinforcement. Although not currently addressed by ACI Committee 318, fibers are sometimes used in structural applications with conventional reinforcement. American Concrete Institute, ACI 318 Building Code Requirements for Reinforced Concrete, 1995.
The practice of adding steel fibers to concrete to overcome its drawbacks was first introduced early in this century. Between 1920 and 1935 several patents pertaining to steel fiber reinforced concrete (SFRC) were granted. See, e.g., Kleinlagel, A, German Patent No. 388.959; Scailles, J. C., French Patent No. 514.186; Martin, G. C., U.S. Pat. No. 1,633,219; and Etherridge, H., U.S. Pat. No. 1,913,707. Use of glass fibers in concrete was first attempted in the USSR in the late 1950s. Biryukovich, K. L., and Yu, D. L., "Glass Fiber Reinforced Cement," (translated by G. L. Cairns, CERA Translation, No.12, Civil Eng. Res. Assoc., London, 1965). Initial attempts at using synthetic fibers (nylon, polypropylene) were made in the 1960s. Monfore, G. E., "A review of Fiber Reinforced Portland Cement Paste, Mortar and Concrete," J Res. Dev. Labs, Vol. 10, No. 3, September 1968, pp. 36-42; Goldfein, S., "Plastic Fibrous Reinforcement for Portland Cement," Technical Report No. 1757-TR, U.S. Army Research Development Laboratories, Fort Belvoir, October 1963, pp. 1-16.
When steel fibers first were used, only straight steel fibers were employed. The use of steel fibers resulted in improved characteristics for ductility and fracture toughness; flexural strength increases were also reported. For straight steel fibers, the primary factors that controlled the properties of the composite were fiber volume fraction and length/diameter, or aspect ratio of the fibers. The amount of fibers ranged from 90 to 120 kg/m.sup.3 (1.1 to 1.5% by volume) of concrete. The aspect ratios were in the range of 60 to 100. The major problems encountered in the early stages were difficulty in mixing and workability. At higher volume fractions, fibers were found to ball up during the mixing process. This process, called bailing, was found to occur frequently for longer fibers. This tends to affect the quality of concrete in place, especially for higher fiber volume fractions. Furthermore, there was always a reduction in workability of the concrete as a result of the addition of fibers.
The advent of deformed steel fibers in the late 1970's resulted in increased use of fiber reinforced concrete in the field. Ramakrishnan established that fibers with hooked ends can be used at much lower volume fractions than straight steel fibers, producing the same results in terms of product ductility and toughness. Ramakrishnan, V., Brandshaug, T., Coyle, W. V., and Schrader, E. K., "A Comparative Evaluation of Concrete Reinforced with Straight Steel Fibers and Deformed End Fibers Glued Together in Bundles," ACI Journal, Vol. 77, No.3, May-June 1980, pp. 135-143. These fibers were glued together at the edges with water soluble glue such that, when added to the concrete, the fibers had a much lower (apparent) aspect ratio. Duing mixing, the fibers were separated and dispersed as individual fibers. The gluing and subsequent dispersal, in combination with a lower volume fraction of fibers, resulted in virtual elimination of balling. Later, a number of other fiber shapes such as crimped, paddled, and enlarged ends were also developed.
The ACI 544 Committee Report on Fiber Reinforced Concrete, published in 1996, reports that the first significant use of synthetic fibers in concrete was done in 1965, by the U.S. Army Corps of Engineers Research and Development Section. American Concrete Institute ACI 544.1R-96, "Fiber Reinforced Concrete," 1996. Synthetic monofilament fibers were used for construction of blast-resistant concrete structures. The fibers used were 13 to 25 mm long and had an aspect ratio of between 50 to 100, i.e. geometry not too different from the steel fibers being used in concrete at that time. With these fibers it was found that addition rates of up to 0.5% by volume of the concrete resulted in significant increases in ductility and impact resistance.
There was, however, very little commercial exploitation of fiber reinforcement technology and it was not until the 1980's that large scale development and use of synthetic fibers in concrete started to take place. This work was predominantly done with much lower denier fibers (i.e. small diameter fibers with high aspect ratios) at lower fiber addition rates. Morgan, D. R, and Rich, L., "High Volume Synthetic Fiber Reinforced Shotcrete," The First International Conference on Synthetic Fiber Reinforced Concrete, Orlando, Fla., USA, Jan. 16, 1998. Most work was performed with collated, fibrillated polypropylene fibers at 0.1 to 0.2% by volume addition rates. At these lower fiber volume addition rates, the primary benefits of the fibers are for plastic shrinkage crack control and provision of green strength to extruded and certain green-cast, precast concrete products. Enhancement of ductility and impact resistance, and resistance to long term restrained drying shrinkage cracking is limited at such low fiber volume addition rates. It should be noted that even at these low fiber addition rates, the fiber count (number of fibers in a unit volume of matrix) and specific surface (surface area of fibers per unit volume of matrix) is very high. Consequently, it is currently very difficult to introduce in concrete more than 0.4% by volume of conventional fibrillating polypropylene fibers without making significant changes to the concrete mixture design. As a result, the majority of synthetic fibers used today are incorporated in concrete at very low fiber addition rates to simply control plastic shrinkage.
With the emergence of new areas of application, research interest has moved to higher fiber contents where toughness index and other factors are design considerations. Toughness index is an indication of the load-carrying capabilities of the fibers within the concrete matrix after first crack. As previously mentioned, cast-in-place concrete will accommodate up to 0.4% by volume of synthetic fibers with minimal mix proportion adjustments. Wet mix shotcrete with fibers added at a rate of up to 0.75% by volume will provide major increases in toughness index values. Morgan, D. R, McAskill, N., Richardson, B. W., and Zellers, R. C., "A Comparative Evaluation of Plain, Polypropylene Fibers, Steel Fibers, and Wire Mesh Reinforced Shotcrete," Transportation Research Board, Washington D.C., January 1989. Fiber length and fiber configuration are important factors at this fiber content. In slab-on-grade applications, the use of collated fibrillated polypropylene fiber at contents up to 0.3% by volume has dramatically increased the fatigue strength. American Concrete Institute ACI 544.1R-96, "Fiber Reinforced Concrete," 1996.
A few years ago, a new monofilament, polyolefin fiber, with a unique dispensing system, was developed, which has now been used at fiber addition rates in the 1.0 to 2.0% by volume range (i.e. values up to 10 times greater than the conventional use of fibrillated polypropylene fibers). The fiber has been used in a range of different concrete flatwork and other cast-in-place concrete applications, e.g. full-depth concrete pavement, bridge deck overlays, whitetopping, etc. Ramakrishnan, V., and MacDonald, C. N., "Durability Evaluation and Performance Histories of Projects Using Polyolefin Fiber Reinforced Concrete," ACI British Columbia Chapter, High Performance Concrete Seminar, Vancouver BC, April 1997, p. 15. The fibers used range in length from 25 to 50 mm and have aspect ratios in the range of 66 to 80. At these much higher fiber addition rates, ducility, impact resistance and toughness in the concrete composites are substantially increased and are much more comparable to values achieved with steel fiber reinforced concretes, with 0.5 to 0.7% (40 to 55 kg/m.sup.3) fiber addition by volume. Apart from demonstrating excellent reinforcing characteristics the polyolefin fiber has the advantage over its steel counterpart in that it would never rust. Following the same philosophy (use of polymeric fiber at high addition rates), Synthetic Industries has just released a new polymer fiber for shotcrete applications. The new fiber, called S-152 High Performance Polymer (HPP), is manufactured as a coarse filament with an engineered contoured profile. Synthetic Industries, product literature, 1998. The wave-like shape of the fibers is designed to anchor the fibers in the shotcrete. Additionally, the coarseness of the fiber allows it to be blended at much higher rate per unit volume than conventional fibers, thereby giving the shotcreted application enhanced structural performance.
Polyolefin fiber is, however, of limited utility because it is a monofilament fiber that will remain in its original form after mixing. It has a relatively low surface area and hence has correspondingly poor bonding characteristics. Therefor, one must introduce a relatively large percent by volume (1.5% and higher) in order to achieve beneficial results.
Addition of fibers to concrete will generally result in loss of slump and handling characteristics of the mixture. This loss is magnified as the aspect ratio length/diameter) of the fiber or the addition rate of fibers increases. For conventionally mixed fiber reinforced concrete, high aspect ratio fibers are more effective in improving the post-peak performance because of their high resistance to pullout from the matrix. A detrimental effect of using high aspect ratio fibers is the potential for balling of the fibers during mixing.
Most synthetic fibers used today are fibrillated fibers having very high surface areas and aspect ratios. The very high surface areas of these fibers make it very difficult to produce a workable concrete mixture at higher fiber addition rates than 0.5% by volume, without causing severe workability and fiber balling problems. For this reason, synthetic fibers are mostly used at 0.1% by volume fiber addition rates, and are added mostly for the control of shrinkage cracking in concrete.
Thus, there is still a need in the art for fiber reinforced concrete formulations that overcome the disadvantages noted above, while still maintaining superior shrinkage and handling characteristics. More particularly it would be desirable to be able to employ fiber of a type that is less sensitive to balling and can therefor be added at higher volume fractions. Higher volume fractions will result in increased reinforcing characteristics, previously unattainable with fiber-reinforced building materials such as concrete.